1. Field of the Invention
The present invention relates to high-voltage generating circuits, and more particularly, to a high-voltage generating circuit for generating a high voltage applied to the anode of a cathode-ray tube (CRT).
2. Description of the Related Art
A high-voltage generating circuit as shown in FIG. 10 is conventionally known. This high-voltage generating circuit 141 generally includes a high-voltage production circuit 160, a high-voltage detecting circuit 161, and a control circuit 162. The high-voltage detecting circuit 161 detects a fluctuation of a high voltage output from the high-voltage production circuit 160. The control circuit 162 controls a variable DC power source 146 according to a detection voltage output from the high-voltage detecting circuit 161 to compensate the high voltage output from the high-voltage production circuit 160 for its fluctuation and to make the high voltage stable.
The high-voltage production circuit 160 is formed of a switching device 142, a damping diode 143, a resonant capacitor 144, a flyback transformer 145, the variable driving power source 146, and rectifying diodes 156 and 157.
The high-voltage detecting circuit 161 includes voltage-dividing resistors 147, 148, 149, and 150 for dividing the high voltage output from the high-voltage production circuit 160, and speed-up capacitors 151 and 152 for increasing the rise speed of the high voltage output from the high-voltage production circuit 160. Voltage-dividing resistors 147, 148 and 149 are connected in series, and together with the speed-up capacitor 151, form a parallel circuit serving as a high-voltage circuit section 161a. The voltage-dividing resistor 150 and the speed-up capacitor 152 form a parallel circuit serving as a low-voltage circuit section 161b. The voltage-dividing resistor 148 also serves as a focus voltage adjustment resistor, and the voltage-dividing resistor 149 also serves as a screen voltage adjustment resistor. There are also shown a focus capacitor 154 and a screen capacitor 155.
Since a high voltage (1 kV to several tens of kilovolts) output from the high-voltage production circuit 160 is applied to the speed-up capacitor 151 in the conventional high-voltage generating circuit 141, there has been a problem that the capacitor 151 needs to satisfy a high breakdown voltage specification, and is expensive and large.
In addition, a cathode-ray tube (CRT) generally has a capacitance of several hundreds to several thousands of picofarads. When a screen A of the cathode-ray tube has a white portion W having a high luminance level (hatched portion B is black) as shown in FIG. 11, a large beam current flows at the white portion W in the cathode-ray tube and a high voltage cannot quickly be reproduced if the high voltage drops. As a result, the contour of the white portion W on the screen is distorted as shown by dotted lines, which is called a trapezoidal distortion, and the image quality deteriorates. It has been proposed as a countermeasure to use the speed-up capacitor 151 having a large capacitance. However, in the case, it is necessary to extend the blanking period so as not to clearly show a change on the screen until the high voltage rises because the rise speed of a high voltage decreases. Although a smoothing capacitor having a large capacitance and satisfying the high breakdown voltage specification may be provided in addition to the speed-up capacitor 151, the additional capacitor is expensive and large, whereby the high-voltage generating circuit becomes large and expensive.